The present invention, in some embodiments thereof, relates to radiopharmaceuticals and, more particularly, but not exclusively, to novel radiolabeled EGFR-TK inhibitors and their use in radioimaging (e.g., PET or SPECT) and in radiotherapy.
Polypeptides such as growth factors, differentiation factors, and hormones often mediate their pleiotropic actions by binding to and activating cell surface receptors with an intrinsic intracellular protein tyrosine kinase activity. The epidermal growth factor receptor (EGFR) is one such receptor.
Receptor tyrosine kinases are large enzymes which span the cell membrane and possess an extracellular binding domain for growth factors such as the epidermal growth factor, a transmembrane domain, and an intracellular portion which functions as a kinase to phosphorylate specific tyrosine residues in proteins and hence to influence cell proliferation. It is known that such kinases are frequently aberrantly expressed in common human cancers. It has also been shown that the epidermal growth factor receptor (EGFR) is mutated and/or overexpressed in many human cancers such as brain, lung, squamous cell, bladder, gastric, breast, head and neck, oesophageal, gynecological and thyroid tumors.
Some mutation variants in the tyrosine kinase domain of the EGFR gene result in increased activity of the tyrosine kinase and in constitutive activity of the receptor, resulting in uncontrolled cell proliferation. Such mutation variants are commonly termed “activating mutations”, and are observed, for example, in patients having non-small cells lung cancer (NSCLC). Activating mutations in the tyrosine kinase domain of the EGFR gene typically confer sensitivity to EGFR tyrosine kinase small molecule inhibitors, whereby other mutation variants in the EGFR may result in insensitivity or resistance to EGFR-TK small molecule inhibitors.
EGFR small molecule tyrosine kinase inhibitors (abbreviated as EGFR-TKIs or simply as TKIs) bind to the tyrosine kinase domain of the EGFR on the cytoplasmic side of the receptor, and inhibit its tyrosine kinase activity. Without kinase activity, the EGFR is unable to further bind to and activate downstream proteins. By interfering with (halting) the signaling cascade in cells that rely on this pathway for growth, cell proliferation, survival and migration are diminished. EGFR-TKIs are therefore considered a selected therapy in the presence of activating mutation in the tyrosine kinase domain of the EGFR gene, as, for example, in some cases of NSCLC.
When aberrant cell proliferation is associated with deregulated expression and/or activity of EGFR that does not result from an activating mutation in the tyrosine kinase domain of the EGFR gene, the treatment regime typically utilizes, for example, compounds which inhibit DNA synthesis. Such compounds are known as cytotoxic agents, and are disadvantageously characterized by adverse side effects due to their non-selectivity.
Erlotinib, also known as Tarceva™, or N-(3-ethynylphenyl)-6,7-bis(2-methoxyethoxy)quinazolin-4-amine, is a reversible EGFR-TKI, featuring the following chemical structure:

Erlotinib is described, for example, in U.S. Pat. Nos. 5,747,498 and RE41065, which are incorporated by reference as if fully set forth herein.
Erlotinib has a 4-anilinoquinazoline skeleton, and features an ethynyl substituent on the aniline ring and methoxyethoxy (Methoxy-terminated ethylene glycol) substituents at positions 6 and 7 of the quinazoline ring.
Currently, erlotinib is approved for the treatment of NSCLC and pancreatic cancer.
Lung cancer accounts for almost 30% of cancer-related deaths, with non-small cell lung cancer (NSCLC) representing approximately 80% of lung cancer cases.
There are currently various first-line therapeutic approaches for patients with advanced NSCLC. Since over-expression of the epidermal growth factor receptor (EGFR) has been detected in the majority of NSCLC tumors, this tyrosine kinase (TK) receptor emerged as the focus of various targeted therapeutic approaches. EGFR-TK selective small molecule inhibitors (EGFR-TKIs), such as gefitinib and erlotinib, gained FDA approval approximately 12 years ago, for treating advanced NSCLC.
It has been recognized that a successful treatment of NSCLC is largely determined by the histopathological and molecular characteristics of the tumors. Clinical experience has taught that only patients whose tumors harbor activating mutations in the TK domain of the EGFR gene, such as the del(E746-A750) and the L858R mutations, benefit from TKI-therapy over cytotoxic chemotherapy. Therefore, at present, first-line treatment of NSCLC patients using EGFR-TKIs is recommended only for patients whose tumors harbor such activating mutations [Carnio et al. Semin Oncol 2014, 41:69-92; Dillon et al. Lancet Oncol 2012, 13:764-765].
Activating mutations in the TK domain of the EGFR gene occur in 10-30% of NSCLC patients [Ratti M. and Tomasello G. Anticancer Drugs 2014, 25:127-139], and evidence of their presence is a prerequisite for the initiation of first-line targeted therapy with selective EGFR-TKIs, such as erlotinib.
At present, the identification and selection of NSCLC patients who are candidates for first line treatment with selective EGFR-TKIs require biopsy of the primary tumor and further analysis by genotyping and/or immunohistochemistry (IHC) of tissue specimens for verifying the mutational status of the EGFR. Various techniques of obtaining lung biopsies exist, which are invasive and costly, necessitate tissue samples of sufficient quality, and require time for mutation analysis. Moreover, the histopathological and molecular characteristics of tissue specimens retrieved from the primary tumor do not necessarily represent those of distant metastases, nor do they provide information about their presence and location.
Furthermore, the majority of TKI-treated NSCLC patients ultimately develop resistance to treatment, with the most common mechanism of resistance involving the emergence of the secondary gate-keeper T790M mutation in exon 20 of the EGFR gene [Riely et al. Clin Cancer Res 2007, 13:5150-5155; Yu et al. Clin Cancer Res 2013, 19:2240-2247]. Thus, monitoring response to treatment during TKI-therapy is essential, since adjustments and modifications of the treatment approach throughout the course of treatment may be required. However, similarly to the process of patient selection prior to treatment with TKIs, identification of patients whose tumors harbor the secondary T790M mutation typically requires interfering procedures such as biopsy. Tumor biopsies, however, are less applicable for longitudinal monitoring of the EGFR's mutational status during the course of treatment. Thus, the prevailing approach of patient selection is not optimal for obtaining longitudinal information about the molecular characteristics of EGFR in tumors.
The use of radioactive nuclides for medicinal purposes is well known in the art. Biologically active compounds that bind to specific cell surface receptors or modify cellular functions have received some consideration as radiopharmaceuticals, and therefore, when labeled with a radioactive nuclide, such compounds are used as biospecific agents in radioimaging and/or radiotherapy.
Positron emission tomography (PET), a nuclear imaging technique which allows the three-dimensional, quantitative determination of the distribution of radioactivity within the human body, is widely recognized as an important tool for the measurement of physiological, biochemical, and pharmacological function at a molecular level, both in healthy and pathological states. PET involves the administration of a molecule labeled with a positron-emitting nuclide (radiotracer) such as 15O, 13N, 11C and 18F, which have half-lives of 2, 10, 20, and 110 minutes, respectively.
Similarly, single photon emission computed tomography (SPECT) is a form of nuclear imaging, in which emissions from radioactive compounds, labeled with gamma-emitting radionuclides, are used to create 3D images of radioactivity distribution in vivo. SPECT requires the administration of a molecule labeled with a gamma-emitting nuclide, such as 99mTc, 67Ga, 111In and 123I.
Radiotracers that bind to EGFR-TK and thereby allow, through a nuclear imaging technique, such as PET, mapping and quantification of this receptor-kinase, and detecting changes in its levels of expression, and optionally, depending on the radiotracer used, can be utilized also in radiotherapy, have been disclosed, for example, in U.S. Pat. Nos. 6,126,917, 6,562,319, 7,172,749, and 8,461,166.
Non-invasive molecular imaging techniques such as positron emission tomography (PET) have been suggested for identifying EGFR's mutational status in tumors in order to determine if patients are expected to be responsive to TKI-therapy, prior to treatment, and to monitor the EGFR's mutational status during the course of treatment. A use of [11C]erlotinib PET for identifying NSCLC tumors that harbor exon-19 in-frame deletions in human subjects and in mouse models, has been described in Bahce et al. Clin Cancer Res 2012, 19:183-193; and Weber et al. J Thorac Oncol 2011, 6:1287-1289, for human subjects, and in Memon et al. Cancer Res 2009, 69:873-878; and Petrulli et al. Neoplasia 2013, 15:1347-1353, for mice models.
Recently, the ability to differentiate erlotinib-responsive NSCLC tumors in mice from non-responsive or resistant tumors, using [11C]erlotinib PET, has been reported. See, Abourbeh et al. EJNMMI Res 2015, 5:4.
Additional background art includes U.S. Pat. No. 8,575,339, which describes, inter alia, halo (e.g., fluoro)-containing derivatives of erlotinib; and PCT International Patent Application Publication No. WO 2014/118197, which describes fluorine-18 radiolabeled afatinib analogs for nuclear imaging of EGFR and for determining the mutational status of EGFR.